1. Field of the Invention
The present invention generally relates to a color image forming apparatus which forms a monochromatic toner image or a toner image in a plurality of colors. More particularly, this invention relates to a color image forming apparatus which changes fixing energy depending on whether a toner image is monochromatic or in a plurality of colors.
Due to a demand for image recording on ordinary sheets of paper, image forming apparatuses (such as, a copying machine, a printer and a facsimile) employ a latent-image forming type recording apparatus (e.g., electrophotographing apparatus). Such an image forming apparatus forms an electrostatic latent image on a photosensitive drum and then develops the latent image to form a toner image. After transferring the toner image on the photosensitive drum onto a sheet, the apparatus fixes the toner image on the sheet by heat.
In view of a demand for printing colored images, color image forming apparatuses for forming a toner image in a plurality of colors have been developed and are available. The color image forming apparatuses can form monochromatic toner images, as well as toner images in a plurality of colors, and are thus used to print both types of toner images. As this image forming apparatus uses heat to fix a toner image, most of the power consumed by the apparatus is used for image fixation. Since the fixing process needs fixing energy that is related to the thickness of the layer of toners, greater fixing energy is needed to fix a toner image in a plurality of colors; thus, increasing the amount of power consumption of the apparatus. It is therefore desirable to reduce the amount of power consumption for such a fixing process; and this invention accomplishes such an objective.
2. Description of the Relevant Art
In general, the fixing energy E necessary to fix a toner image essentially includes energy E1 for melting toners, and energy E2 which is absorbed in a sheet. The energy E2, absorbed in a sheet, is the same for the fixing energy E for monochromatic printing and the fixing energy E for color printing. However, the energy E1 required to melt toners depends on the thickness of the toner layer, etc., as apparent from the following equation, and thus, varies in accordance with the thickness of the toner layer: EQU E1=sheet feeding speed.times.sheet width .times.toner layer thickness.times.toners' specific heat .times.toners' specific weight .times.rising temperature of toners/heat efficiency. (1)
Monochromatic printing involves a single toner layer, whereas color printing requires different colors to be placed one on another and thus, involves a plurality of toner layers. For instance, two toner layers are needed for multicolored (seven colors) printing, and four toner layers for full color printing. Therefore, greater energy to melt toners is needed for color printing than for monochromatic printing.
In view of the above, the fixing energy may be set to a constant level that is needed for the fixation of a full-colored image; and this fixing energy is used for the fixation of a monochromatic image. This method, however, results in wasteful power consumption because the energy for fixing a full-colored image is more than four times the energy for monochromatic printing.
In order to overcome these deficiencies, various methods have been proposed to reduce the power consumed by the above type of color image forming apparatus in fixing a toner image in monochromatic printing. The first method, which is disclosed in, for example, Japanese Unexamined Patent Publication No. 70571/1986, changes the set temperature for the heat roller in accordance with the type of printing (e.g., 190.degree. C. for color printing and 180.degree. C. for monochromatic printing).
The second method, which is disclosed in, for example, Japanese Unexamined Patent Publication No. 263173/1985, changes the set temperature and the amount of power for the pressure roller, while keeping the set temperature and the amount of power for the heat roller constant. According to this method, the set temperature and the amount of power for the pressure roller are increased for full-colored printing, and are decreased for monochromatic printing.
The above-described conventional apparatuses and methods have the following drawbacks.
The first conventional method increases the set temperature of the heat roller, located on the toner image side, for full-color printing in order to well melt the entire toner image. This is likely to cause a so-called offset phenomenon in which the toners on the surface of the recording medium stick on the heat roller. Accordingly, the heat roller becomes dirty, thereby staining the recording medium (sheet).
According to the second conventional method, the set temperature for the pressure roller, even if changed, contributes to melting the toners indirectly, but via the sheet, resulting in a lower heat efficiency for melting the toners. The heat roller therefore requires the energy that is necessary to melt the toners of a full-colored toner image. In monochromatic printing, therefore, the consumed power is reduced only by the amount equivalent to the decreased temperature of the pressure roller, and a significant reduction of the consumed power cannot be expected. Further, if the temperature of the pressure roller is changed, the fixed toner image contacts the pressure roller at the time of printing both sides so that the fixed toner image may be melted.
It is therefore an object of the present invention to provide a color image forming apparatus which can considerably reduce power consumption, while preventing an offset at the time of fixing a toner image.
It is another object of the present invention to provide a color image forming apparatus which can reduce the amount of power consumption needed for image fixation in accordance with the ambient temperature.